The present invention relates to a device for removing the cake and/or the residual layer retained against the filtering cloth of a basket of a horizontal- or vertical-axis centrifuge after centrifugation of the substance to be treated.
The cake is constituted by the filtered solid of the substance to be treated, while the residual layer is formed by the filtered solid remaining against the filtering cloth after scraping of the cake in order to avoid deterioration of the cloth by the cutting element of the scraper.
The residual layer is generally removed from the centrifuge either because it hinders filtration between two centrifugations of the same product and this by an effect of clogging, or because a change of substance to be treated renders necessary the complete removal of the product previously decanted.
Devices for removing the residual layer when the centrifuge is either in the off position or driven in rotation, are known.
When the centrifuge is in the off position, the residual layer is removed manually with the aid of a special tool. This intervention is dangerous as the operator comes into contact with the product which may include noxious agents. In addition, this technique is long and expensive and risks polluting the decanted product due to the opening of the machine.
When the centrifuge is driven in rotation, different devices exist for removing the residual layer, such as pneumatic or mechanical accessories. When using pneumatic removal accessories, it will be noted that the force of impact of the gas projected under pressure does not suffice to remove the decanted product completely, due to the low density of the gas injected. On the contrary, when using mechanical removal accessories, or scrapers, it is observed that the contact of the plastic element on the filtering cloth wears this latter, causing losses of cloth in the solid, which requires a constant verification thereof.
According to Japanese Patents JP 59 95954 and JP-60 022955, devices under pressure for removing the filtered solid forming the cake, are known.
Japanese Patent JP 59 95954 shows a tight, elastic bag which is placed between the inner wall of the basket and the filtering cloth. The bag comprises a certain number of holes to allow evacuation of the liquid during centrifugation. A gas under pressure is sent inside the bag to displace the filtering cloth in the direction of the axis of rotation of the machine in order to break the residual layer remaining against the filtering cloth after scraping of the cake.
The device presents certain drawbacks concerning the elastic bag which prevents the passage of a large quantity of liquid inside the machine. In fact, the liquid is retained upstream of the filtering cloth due to the presence of the tight bag which reduces the filtration surface, this considerably degrading drying of the solid layer.
Japanese Patent JP 60 022955 shows a filtering cloth with double thickness so as to constitute a bag which deforms under the action of a fluid under pressure. This device cannot function as it is impossible to construct a bag allowing passage of the liquid phase during spinning and then being able to be inflated and remain tight under a high pressure varying between 3 and 6 bars.